Hematology and Coagulation: A Comprehensive Review for Board Preparation, Certification and Clinical Practice

Summary

Hematology and Coagulation is a clear and easy-to-read presentation of core topics and detailed case studies that illustrate the application of hematopathology knowledge to everyday patient care. In order to be successful, as well as to pass the American Board of Pathology examination, all pathology residents must have a good command of hematopathology, including the challenging topics of hematology and coagulation. Hematology and Coagulation meets this challenge head on.

This basic primer offers practical examples of how things function in the hematopathology clinic as well as useful lists, sample questions, and a bullet-point format ideal for quick pre-board review. This book provides only the most clinically relevant examples designed to educate senior medical students, residents and fellows and "refresh" the knowledge base, without overwhelming students, residents, and clinicians.

Takes a practical and easy-to-read approach to understanding hematology and coagulation at an appropriate level for both board preparation as well as a professional refresher course
Covers all important clinical information found in larger textbooks in a more succinct and easy-to-understand manner
Covers essential concepts in hematopathology in such a way that fellows and clinicians understand the methods without having to become specialists in the field

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Hematology and Coagulation - Amer Wahed

Dasgupta

Preface

Amer Wahed and Amitava Dasgupta, Houston, Texas

This is the second book in a series of books, designed for board review for Pathology residents. The first book in this series Clinical Chemistry, Immunology and Laboratory Quality Control: A Comprehensive Review for Board Preparation, Certification and Clinical Practice was published, also by Elsevier, in January 2014. The aim of this current book is to provide a strong foundation for students, residents, and fellows embarking on the journey of mastering hematology. It is expected that this will also act as a valuable resource for residents preparing for the clinical pathology board exam. Thus, this book should not be considered a conventional textbook of hematology. There are several such textbooks of hematology and most of them are excellent ones. We have deliberately refrained from including pictures, as conventional textbooks and the internet are excellent sources of such images. At the same time, this will allow the final cost of the book to remain reasonable. We have added a section, denoted as Key Points at the end of each chapter. We hope that this section will be a good resource for reviewing information, when time at hand is somewhat limited.

We are confident that medical students with a keen interest in hematology will find this book to be of benefit, as well as clinical hematologists who wish to have a better understanding of the diagnostic aspects of hematologic diseases.

We would like to thank our Department Chair, Dr. Robert Hunter, for encouraging us to write the second book in the series. We also thank our pathology residents who read various chapters of this book critically and made extremely helpful comments. In particular, we have to mention Dr. Alyaa Al-Ibraheemi, Dr. Andres Quesada, and Dr. Elizabeth Jacobi in this regard. If readers enjoy reading this book, our efforts will be duly rewarded.

Chapter 1

Complete Blood Count and Peripheral Smear Examination

This chapter discusses accurate and appropriate interpretation of complete blood counts (CBCs) as well as the reviewing of peripheral smears. Such review is important not only to detect benign hematological abnormalities but to detect hematological malignancies. Technical aspects of hematology analyzers are also discussed, As are common challenges in CBC interpretation and peripheral smear review examination.

1.1 Introduction

A complete blood count (CBC) is one of the most common laboratory tests ordered by clinicians. Even for a routine health checkup of a healthy person, CBC is ordered to ensure there is no underlying disease when the individual may be asymptomatic. Tefferi et al. noted that at the Mayo Clinic in Rochester, Minnesota, approximately 10–20% of CBC results are reported as abnormal. Common abnormalities associated with abnormal CBC include anemia, thrombocytopenia, leukemia, polycythemia, thrombocytosis, and leukocytosis [1]. For CBC analysis, the specimen must be collected in an ethylene diamine tetraacetic acid (EDTA) tube (lavender or purple top).

CBC consists of numbers that are printed out from the hematology analyzer. In addition, the printout contains graphs and flags. Flags are essentially messages provided by the analyzer to the interpreting person that certain abnormalities may be present. For example, an analyzer may flag that blasts are present. Therefore, a review of blood smear slides is required to verify the presence of blasts. To make a meaningful interpretation of the peripheral smear, a review of not only the CBC printout but also the patient’s electronic medical records must be conducted. CBC parameters that are printed from an automated hematology analyzer are red blood cell (RBC)-related numbers, white blood cell (WBC)-related numbers, and platelet-related numbers (Box 1.1).

Box 1.1

Various Parameters Printed by a Hematology Analyzer Following CBC Analysis

Red Blood Cell (RBC)-Related Numbers

RBC count

Hemoglobin level

Hematocrit

Red cell differential width (RDW)

Mean corpuscular volume (MCV)

Mean corpuscular hemoglobin (MCH)

Mean corpuscular hemoglobin concentration (MCHC)

Reticulocyte count

White Blood Cell (WBC)-Related Numbers

Total WBC count corrected

Total WBC count uncorrected

WBC differential

Absolute count of each type of WBC

Platelet-Related Numbers

Platelet count

Mean platelet volume (MPV)

Platelet differential width (PDW)

1.2 Analysis of Various Parameters by Hematology Analyzers

Modern hematology analyzers are capable of counting as well as determining the size of various circulating blood cells in blood, including RBCs, WBCs, and platelets [2]. One such instrument, the Beckman–Coulter analyzer, generates an electrical pulse when a blood cell passes through the analyzer channel, which consists of a small aperture surrounded by electrodes. Each electrical pulse represents an individual cell, and pulse height indicates the cell volume. Modern hematology analyzers are also capable of multimodal assessment of cell size and cell count, thus providing additional information regarding various categories of WBCs, such as neutrophils, lymphocytes, monocytes, eosinophils, and basophils.

The following are examples of various channels in a hematology analyzer:

Channel for red cells (and also platelets): This channel is capable of analyzing red blood cells and platelets.

Different hematology analyzers may use different methods for counting, including the following (one analyzer may employ multiple methods):

Impedance

Conductivity measurements with high-frequency electromagnetic current (depends on the internal structure, including nuclear cytoplasmic ratio and nuclear density to granularity ratio)

Light scatter

Fluorescence-based methods.

1.2.1 RBC Count and Hemoglobin Measurement

Typically, one channel is used to detect RBCs and platelets. The detector is set such that any cell between 2 and 30 fL will be counted as a platelet and any cell between 40 and 250 fL will be counted as a red cell. If there are large platelets, these will be counted as red cells and will also result in a falsely low platelet count. Similarly, if there are fragmented red cells, these smaller red cells will be counted as platelets.

For hemoglobin measurement, the spectrophotometric method is used after the red cells are lysed. The principle of the method is oxidation of ferrous ion of hemoglobin by potassium ferricyanide into the ferric ion of methemoglobin, which is then converted into stable cyanomethemoglobin by potassium cyanide. However, sulfhemoglobin, if present, is not converted into cyanomethemoglobin under this reaction condition. Usually, dihydrogen potassium phosphate (added to lower pH and accelerate the reaction) is used in the reaction mixture. Nonionic detergents are also used to accelerate lysis and reduce turbidity. Finally, absorbance of light at 540 nm is measured, and the intensity of the signal corresponds to hemoglobin concentration.

Smokers have a higher than normal carboxyhemoglobin concentration because carboxyhemoglobin takes longer to convert into cyanomethemoglobin and also absorbs more light at 540 nm compared to cyanomethemoglobin. Thus, the hemoglobin value in smokers may be falsely elevated. Nordenberg et al. commented that cigarette smoking seems to cause a generalized upward shift of the hemoglobin distribution curve, which reduces the diagnostic value of detecting anemia in smokers using the hemoglobin value. The authors suggested that the minimum hemoglobin cutoff level for anemia should be adjusted for smokers [3]. Fetal hemoglobin may interfere with spectrophotometric measurement of carboxyhemoglobin, thus falsely indicating carbon monoxide poisoning in an infant [4].

Hyperlipidemia and hypergammaglobulinemia can also falsely elevate hemoglobin levels. In cold agglutinin disease, red cell agglutination usually takes place. In such situations, a clump of red cells may be counted as one red cell. Thus, the RBC count may be falsely low, and the mean corpuscular volume (MCV) will be falsely high. However, when the red cells are lysed, a true hemoglobin result will be available. Therefore, a clue to cold agglutinin disease is a disproportionate low RBC count compared to the hemoglobin level.

In the context of RBC parameters, measurement of MCV and red blood cell distribution width (RDW) is also performed. The hematocrit (Hct) value is calculated from the MCV and the RBC count using the following formula (normal Hct value approximately 45%):

RDW is a measure of the degree of variation of size of red cells—that is, it reflects the extent of anisocytosis. RDW is elevated in iron deficiency anemia, myelodysplastic syndrome, and macrocytic anemia secondary to vitamin B12 or folate deficiency. In contrast, RDW is usually normal or mildly elevated in thalassemia. The RBC histogram is plotted using volume as the x axis and percentage as the y axis. The RBC histogram has an ascending slope, a peak, and a descending slope. A perpendicular line drawn from the peak down to the x axis represents the MCV. Mean corpuscular hemoglobin (MCH) refers to the average amount of hemoglobin found in RBCs. Mean corpuscular hemoglobin concentration (MCHC) represents the concentration of hemoglobin in RBCs. Both MCH and MCHC are also calculated values (Box 1.2). In cold agglutinin disease, the RBC count is low, and therefore the Hct is also low; the MCHC is high. The laboratory scientist uses abnormally high MCHC as an indicator of possible cold agglutinin disease and warms the blood prior to repeating the CBC run on the analyzer. In hyperosmolar states, cells swell, causing increased MCV.

Box 1.2

Various Formulae Used for Calculating Different Parameters in CBC Analysis

Hematocrit (Hct)

Mean Corpuscular Hemoglobin (MCH)

Mean Corpuscular Hemoglobin Concentration (MCHC)

Corrected Reticulocyte Count

Reticulocyte Production Index (RPI)

MCH is decreased in patients with anemia caused by impaired hemoglobin synthesis. MCH may be falsely elevated in blood specimens with turbid plasma (usually caused by hyperlipidemia) or severe leukocytosis.

MCHC is decreased in microcytic anemias in which the decrease in hemoglobin mass exceeds the decrease in the size of the RBCs. It is increased in hereditary spherocytosis and in patients with hemoglobin variants, such as sickle cell disease and hemoglobin C disease.

1.2.3 Reticulocyte Count

Reticulocytes are immature red cells. They are named as such because they contain reticular material that is actually RNA. The RNA can be seen with special stains such as new methylene blue. Reticulocyte count is used to assess bone marrow response to anemia. It is important to use the corrected reticulocyte count when making such assessments. The formula is provided in Box 1.2. Reticulocytes may also be assessed using the reticulocyte production index (RPI), and the formula is also provided in Box 1.2.

1.2.4 WBC Count and Differential

The WBC histogram has three peaks. The first peak corresponds to lymphocytes, and the third peak corresponds to neutrophils, whereas the second peak corresponds to the remaining types of WBCs. When nucleated RBCs are present, these cells may be counted as WBCs, especially lymphocytes. The total WBC count may thus be falsely elevated. For an accurate WBC count, the analyzer must be run in the NRBC mode. This is referred to as the corrected WBC count. In some printouts, UWBC represents uncorrected WBC count, and WBC represents corrected WBC count. Also in some printouts, the corrected WBC count is denoted by the sign & before the WBC count. When significant myeloid precursors are present, the downward slope of the neutrophil peak might not touch the baseline.

Pseudothrombocytopenia is an important issue. Causes of falsely low platelet count include the following:

Traumatic venipuncture and activation of clotting

A significant number of large platelets (platelets being counted as RBCs)

EDTA-induced platelet clump

EDTA-dependent platelet satellitism (platelets form a satellite around neutrophils).

The last two conditions are typically diagnosed when the slide is reviewed, although hematology analyzers are capable of flagging platelet clumps. Blood should be re-collected in citrate or heparin. If thrombocytopenia is due to peripheral destruction or consumption of platelets, then the bone marrow responds to the thrombocytopenia by releasing immature platelets, which are larger than normal. This increases the mean platelet volume (MPV) and also the platelet differential width (PDW). If thrombocytopenia is due to reduced production by the bone marrow, large platelets are not seen and thus MPV and PDW are not increased.

1.3 Review of Peripheral Smear

A microscopic examination of appropriately prepared and well-stained blood smear slides by a pathologist or a knowledgeable laboratory professional is useful for clinical diagnosis. A blood smear analysis takes into account flagged automated hematology results and enables the determination of whether a manual differential count should be performed. Therefore, peripheral blood smear examination along with manual differential leukocyte count (if necessary) and CBC provide the complete hematological picture of the patient [5]. Review of the smear should start by ensuring that the name and accession number on the slide match those on the CBC printout. Sometimes naked eye examination of the slide may provide some important clues. For example, if the slide appears blue, there is a possibility of underlying paraproteinemia or myeloma. When paraproteins are present in significant amounts in blood, they are stained blue by the Wright–Giemsa stain. Sometimes tumor emboli are visible as clumps on the slide. Cryoglobulinemia may appear as blobs on the slide.

The slide should be scanned at first at low power to assess overall cellularity (especially of white cells), to find an appropriate area where red cell morphology is best assessed (under higher power), and to check for platelet clumps. Red cell morphology is typically best assessed where the cells are evenly distributed, and this area is away from the tail, toward the body. Rouleaux formation and red cell agglutination can also be appreciated under low power. With experience, blasts can also be picked up on low power. It is then best to assess each cell line under higher power.

Red cells are assessed for size, shape, anisocytosis, central pallor, and red cell inclusions, if any. WBCs should be checked for reactive (toxic) changes, left shift, the presence of blasts, and the degree of segmentation as well as the presence of dysplasia. Platelets should be checked for clumps, size, and adequacy of granules.

1.3.1 Red Cell Variations and Inclusions

Normal red cells are normocytic normochromic. This means that the average size of a red cell in an adult is the size of the nucleus of a mature lymphocyte. Only one-third of the central portion of the red cell has central pallor. Increased pallor means that the red cell is hypochromic. Thus, red cells may be the following:

Normocytic normochromic

Microcytic hypochromic

Macrocytic.

Variation in shape refers to poikilocytosis. Examples of the types of cell found in poikilocytosis are sickle cells, target cells, ovalocytes, elliptocytes, stomatocytes, echinocytes (Burr cells), acanthocytes, schistocytes, spherocytes, dacryocytes (teardrop red cells), and bite cells. Each of these poikilocytes is associated with one or more underlying clinical conditions and is discussed in Chapter 3. There are also various red cell inclusions that can be observed during peripheral blood smear examination. These are listed in Box 1.3.

Box 1.3

Various Red Cell Inclusions

Howell–Jolly bodies

Pappenheimer bodies

Cabot rings

Basophilic stippling (punctate basophilia)

Heinz bodies

Hemoglobin C crystals

Malarial parasite

Nucleated RBCs

1.3.2 WBC Morphology

Reactive changes are predominantly appreciated in neutrophils and lymphocytes. Reactive neutrophils have prominent azurophilic granules, cytoplasmic vacuoles, and Dohle bodies (blue cytoplasmic bodies). Dohle bodies are named after the German pathologist, Karl Gottfried Paul Dohle, and these bodies represent rough endoplasmic reticulum. Dohle bodies in reactive neutrophils are typically seen at the periphery of the cell. Another condition in which Dohle-like bodies may be seen is May–Hegglin anomaly, in which Dohle-like bodies are randomly distributed throughout the cell. They are devoid of organelles. Rather, they are thought to consist of a mutant form of the nonmuscle myosin heavy-chain protein.

Reactive lymphocytes (also known as Downey cells) may be of three types:

A larger than usual lymphocyte with abundant cytoplasm that appears to surround (or hug) the red cells (Downey type II cell) is the most common type of reactive lymphocyte

A small lymphocyte with nuclear membrane irregularity (Downey type I cell)

When there is neutrophilic leukocytosis, a pathologist may observe the presence of immature myeloid precursors in the peripheral blood. This is referred to as left shift. Rare blasts may also be present. Occasionally, patients may present with very high WBC count and left shift mimicking leukemia, although the process is reactive. This is referred to as a leukemoid reaction. Neutrophilic leukocytosis without reactive changes and with basophilia with or without eosinophilia is suspicious for chronic myelogenous leukemia. Numerous blasts may indicate an acute leukemic process.

When the WBC count is high, smudge cells may be observed, which are distorted white cells produced as an artifact during the process of making the slide. The presence of smudge cells with a high lymphocyte count should raise suspicion for chronic lymphocytic leukemia. Pretreatment (prior to making the slide) with albumin ensures that smudge cells are reduced.

Dysplasia is probably the most difficult feature to establish from the peripheral blood. It is most often assessed in the neutrophils. Normal mature neutrophils have two to five nuclear segments and have fine granules in the cytoplasm. Hypogranulation is a feature of dysplasia. Hyposegmentation and hypersegmentation, if present, may represent dysplasia. A bilobed polymorphonuclear leukocyte (PMN) with hypogranulation is referred to as a pseudo-Pelger–Huët cell. This cell is considered to be dysplastic. Hypersegmented PMN is a PMN with more than five segments. This can be seen in megaloblastic anemia as well as myelodysplastic syndrome. It may also be inherited, without any clinical significance. Here, the majority (>75%) of the neutrophils are hypersegmented.

Benign disorders of WBCs, such as May–Hegglin anomaly and Alder–Reilly and Chediak–Higashi diseases, may all be diagnosed from the peripheral smear. Rarely, cells such as hairy cells representing hairy cell leukemia may be seen. Patients with lymphoma may have lymphoma cells circulating in the peripheral blood. These will appear as atypical lymphocytes—that is, lymphoid cells that are neither mature nor reactive in appearance. The presence of a Barr body, which is a nuclear appendage, denotes the inactivated X chromosome and implies female sex of the patient.

1.3.3 Platelets

The normal platelet count is 150,000–450,000/μL. Thrombocytopenia is defined as platelet count below the 2.5th lower percentile of the normal platelet count distribution. Results of the third U.S. National Health and Nutrition Examination Survey support the traditional value of platelet count below 150,000/μL as the definition of thrombocytopenia, but adoption of a cutoff value below 100,000 may be more practical. Thrombocytopenia is a common hematological finding with variable clinical expression or may reflect a life-threatening disorder such as thrombotic microangiopathy [6]. Thrombocytopenia is also a common hematological abnormality found in newborns [7]. Thrombocytopenias can be broadly categorized as follows:

Thrombocytopenias due to decreased production—congenital and acquired (any cause of bone marrow failure)

Thrombocytopenias due to sequestration—sequestration in hemangiomas (Kasabach–Merritt syndrome).

Congenital thrombocytopenias can be broadly divided into three groups: thrombocytopenia with small platelets, thrombocytopenia with normal-sized platelets, and thrombocytopenia with large platelets.

Thrombocytopenia with small platelets can be associated with Wiskott—Aldrich syndrome, X-linked thrombocytopenia, or inherited macrothrombocytes.

Thrombocytopenia with normal-sized platelets can be due to Fanconi’s anemia, thrombocytopenia with absent radii (TAR syndrome), amegakaryocytic thrombocytopenia, or Quebec platelet disorder.

Thrombocytopenia with large platelets may be associated with Bernard–Soulier syndrome, May–Hegglin anomaly, Sebastian syndrome, Epstein syndrome, Fechtner syndrome, or gray platelet syndrome. Normally, the α granules of the platelets are stained by the Wright–Giemsa stain. The δ granules are not stained. The absence of the α granules will result in gray-appearing platelets (i.e., gray platelet syndrome). The platelets are dysfunctional.

Thrombocytosis is defined as platelet counts higher than 450,000/μL. Thrombocytosis may be due to reactive thrombocytosis (associated with infection, inflammation, neoplasms, or iron deficiency); rebound thrombocytosis following thrombocytopenia; redistributional (e.g., post-splenectomy), myeloproliferative disorders such as essential thrombocythemia; or familial thrombocytosis. Platelet disorders are discussed in greater detail in Chapter 5.

1.4 Special Situations with CBC and Peripheral Smear Examination

There are special situations involving CBC and peripheral blood smear review. Pancytopenia is an important hematological finding in which all three major cells present in blood (RBCs, WBCs, and platelets) are decreased in number. Pancytopenia may not be a disease entity but, rather, a triad of findings that may result from a number of diseases primarily or secondarily involving bone marrow. The severity of pancytopenia determines the course of therapy [8]. Important causes of pancytopenia include the following:

1.4.1 Splenic Atrophy or Postsplenectomy

The absence of spleen is characterized by the presence of Howell–Jolly bodies (in RBCs), acanthocytes, and target cells. There may be transient thrombocytosis and leukocytosis as well.

1.4.2 Microangiopathic Hemolysis

There are three important causes of microangiopathic hemolysis:

TTP (thrombotic thrombocytopenia purpura)

HUS (hemolytic uremic syndrome)

DIC (disseminated intravascular coagulation).

All are characterized by low platelets and the presence of schistocytes in the peripheral smear. Typically, there are numerous schistocytes in TTP and HUS, in contrast to DIC, in which there are lower numbers. In DIC, the coagulation profile (e.g., prothrombin time (PT) and partial thromboplastin time (PTT)) are abnormal. In TTP and HUS, the coagulation profile is typically normal. TTP is a medical emergency and requires urgent therapeutic plasma exchange (TPE).

1.4.3 Leukoerythroblastic Blood Picture

This term refers to the presence of red cell precursors (i.e., nucleated red cells) in the peripheral blood as well as WBC precursors (i.e., left shift with blasts). This may be seen in patients with significant hemolysis or hemorrhage. In such situations, clinicians should also search for teardrop red cells. Leukoerythroblastic blood picture with teardrop red cells may be due to a bone marrow infiltrative process. Anemias due to such bone marrow infiltrative processes are known as myelophthisic anemias. This infiltration may be due to many causes, such as fibrosis, infiltration by tumor, or even leukemia.

1.4.4 Parasites, Microorganisms, and Nonhematopoietic Cells in the Peripheral Blood

Several species of parasites, including malaria parasites, and microorganisms, may be seen in the peripheral blood (Box 1.4).

Box 1.4

Parasites and Microorganisms That May Be Present in the Peripheral Blood Smear

Parasites

Malaria: Malarial parasites (see Chapter 3)

Toxoplasmosis: Trophozoite forms are present in monocytes and may also be seen free from ruptured monocytes.

Babesiosis: Trophozoites are seen within red cells. The trophozoites are ring form, similar to that of Plasmodium falciparum. They have one to three chromatin dots. Sometimes they are pear shaped. The pointed ends of four parasites may come into contact and assume the shape of a Maltese cross, which helps in their identification.

Trypanosomiasis: Flagellated parasites of Trypanosoma brucei or cruzi may be seen in the peripheral smear.

Parasitic Microfilaria

Bancroftian filariasis: Caused by human parasitic roundworm Wuchereria bancrofti. The microfilaria form may be seen in the peripheral blood.

Loiasis: Caused by parasitic worm Loa loa. Again, the microfilaria may be seen in the peripheral blood.

Bacteria

Bartonellosis: Rod-shaped coccobacilli may be seen within the red cells due to infection with bacteria of genus Bartonella.

Borreliosis: Borrelia spirochetes may be seen in the peripheral blood due to Lyme borreliosis (Lyme disease) infection.

Fungi

Individuals with impaired immunity and individuals with indwelling catheters may show fungi in their peripheral blood smear. Candida, Histoplasma capsulatum, and Cryptococcus neoformans may be seen within neutrophils.

Occasionally, nonhematopoietic cells may be seen in the peripheral blood. These include the following:

Epithelial cells

Fat cells

Endothelial cells

Malignant cells—for example, neuroblastoma cells, rhabdomyosarcoma cells, and medulloblastoma cells. These cells may resemble lymphoblasts. The presence of carcinoma cells is referred to as carcinocythemia. This is most often observed with carcinoma of the lung and breast. Melanoma cells and Reed–Sternberg cells have been described in the peripheral blood.

1.4.5 Buffy Coat Preparation

Buffy coat films are sometimes made to concentrate nucleated cells (i.e., white cells). This is done to search for low-frequency abnormal cells or bacteria or other microorganisms.

Key Points

For hemoglobin measurement, the red cells should be lysed first. Then hemoglobin (and methemoglobin and carboxyhemoglobin) is converted to cyanomethemoglobin (sulfhemoglobin is not converted). The absorbance of light at 540 nm is then measured.

Smokers have higher than usual carboxyhemoglobin. Carboxyhemoglobin takes longer to be converted to cyanmethemoglobin. Carboxyhemoglobin absorbs more light at 540 nm than does cyanomethemoglobin, giving rise to a falsely higher hemoglobin level.